Softbody simulation on a character
I looked into your file and made some changes of my own.
I noticed in your Softbody settings, under Goal you've selected vertex group: Soft
The issue was that you selected every vertex and weighted them to 1.0. I changed it so the belly part is at 0 weight and tweaked some of the settings and it seems to work for me
I would also recommend to first getting used to softbody physics as Jaroslav advised.